Safer to fly than drive?

We have often heard that, statistically speaking, it is safer to fly commercial airlines than to drive. How would one measure this? How is it measured actually and is it meaningful? And if anyone has link to the actual numbers, that would be great.

This is my stab at it. Pick a sample period of time. Calculate the total number of hours flown, that is, the total time spent in the air by all people. (100 people on a 1 hour flight would contribute 100 hours to the total.) Actually, since accidents occur on the runway too, that should be time spent aboard a commercial plane, not just in the air.

Now do the same calculation for total time spent in autos. Then divide each by the total number of fatalities in airplanes/cars during that time period. If S_F is the time-sum for flying, N_F the number of fatalities, and S_D and N_D the corresponding numbers for driving, then

[tex]\frac{\frac{N_F}{S_F}}{\frac{N_D}{S_D}}[/tex]

would be a ratio of your chance of being killed in an airplane per unit time vs your chance of being killed in a car crash per unit time. It has to either be per-time or per-mile to reflect that you have more of a chance of getting killed the longer your voyage, but perhaps per-mile is better. That way you can compare: what is safer? Driving from NY to LA, or flying from NY to LA?

You could do it for time (fatalities per passenger hour flown) or for distances (fatalities per passenger km/mile flown) or journeys (fatalities per passenger flight).

Distance tends to make planes look safer, since flying 10,000km over open ocean is fairly safe compared to making 1000 x 10km local drives, same for time.
If you want to make planes look dangerous compare accidents per km driven vs accidents/km of a plane on a runway!

Yeah and a meteorite is the most dangerous of all, because statistically the chance of getting killed by a meteorite is bigger than that of getting killed in a plane crash.

[Assuming a small probability for a meteor impact, but a huge fatality number, of course].
Which just goes to show that such "popularized" statistics (and indeed, any statistics, if one is not very careful) can be manipulated to support any conclusion you like.